Microvascular Responsiveness to Cardiopulmonary Bypass
Abstract
Cardiopulmonary bypass can result in multiple organ failure due to mechanisms of ischemia reperfusion injury and the systemic inflammatory response syndrome. The primary objective of this thesis was to investigate and monitor the microvasculature in cardiac surgery patients using multiple methodologies and real-time monitoring techniques. The purpose of our first study was to determine whether pulsatile blood flow during bypass improves microvascular perfusion compared to non-pulsatile flow. We found that changes in sublingual mucosal microcirculation using orthogonal polarization spectral imaging correlate with indices of thenar muscle tissue oxygen saturation and its recovery during a vascular occlusion test using near-infrared spectroscopy in both groups. There were significantly fewer normally perfused vessels, along with impaired microvascular responsiveness and elevated levels of lactate in the non-pulsatile group. Although these technologies help to better understand the pathophysiology of acute circulatory failure, a need exists for improved monitors that can continuously track real-time changes in the microcirculation. Our subsequent studies involved the application of a custom broadband continuous wave near-infrared monitor to determine the feasibility of tracking microvascular hemoglobin content as a surrogate for red blood cell (RBC) flow in skeletal muscle during non-pulsatile bypass. We measure changes in optical density at the isosbestic wavelength as an index of change in hemoglobin over time. The changes in optical density relative to baseline values were continuously monitored throughout the procedure, and showed a positive correlation with various interventions during bypass and with potentially negative outcomes. In our third study we applied continuous wavelet transform analysis to the near-infrared data to reflect the dynamic variability in RBC distribution within the microvasculature as an indicator of autoregulation. We showed signal power composition varied within and between patients at all time points, and shifting of power distribution from high to low frequency ranges, and vice versa, in relation to specific events during the procedure. These studies support the potential for clinical devices that can be easily interpreted by a clinician in real-time to guide therapeutic targets and improve clinical outcomes. Our current research and related future work is an important first step and compelling pre-requisite for such a monitor.